Desirability of measuring friction characteristics of vehicle-travelled surfaces, particularly airport runways, has long been recognized in the art, and a number of apparatus have been developed to fulfill this need. In general, apparatus of this type include a test wheel mounted on a vehicle which is adapted to be propelled over the test surface. In some devices, the test wheel is mounted at an angle to the direction of travel, and surface friction characteristics are measured as a function of forces which tend to align the test wheel with the vehicle direction. In other devices, the test wheel rotates in the direction of travel, and angular velocity of the test wheel is retarded with respect to the vehicle wheels, either at a predetermined fixed "slip" ratio, or at an increasing ratio to wheel skid and lock-up. Surface friction characteristics are measured as a function of forces acting on the test wheel, such as horizontal or braking force, vertical force or load, and brake torque.
French Pat. No. 1,015,251 (1952) discloses an early apparatus in which a test wheel on a self-propelled vehicle is carried on a pivot arm which is selectively lowered by an electric winch into engagement with the test surface at an angle to the direction of vehicle travel. Transducers measure forces on the wheel for determining surface friction characteristics. French Pat. No. 1,476,730 (1967) discloses a similar apparatus wherein test wheels are mounted on respective pivot arms, again on a self-propelled vehicle. The pivot arms and test wheels are coupled to an hydraulic system for selectively lowering the wheels against the test surface and applying a constant vertical load thereto. One wheel is oriented in the direction of travel and may be braked to obtain a predetermined slip with respect to the remaining vehicle wheels. A nozzle directs a film of water beneath the test wheels for simulating wet pavement conditions.
Kullberg, "Method and Equipment for Continuous Measuring of the Coefficient of Friction at Incipient Skid," NAS Highway Research Board Bulletin 348 (1962), pages 18-35, describes a number of early test vehicles constructed by the National Swedish Road Research Institute in Stockholm, Sweden. In general, the various vehicles are self-propelled and employ belt transmissions coupled at variable gear ratio to the vehicle drive system for obtaining fully controlled slip at the test wheels. The test wheels can be raised and lowered selectively by the operator. Vertical wheel load and either horizontal force or wheel torque are measured to obtain friction characteristics, including specifically coefficient of friction, by dividing horizontal force by vertical wheel load. A tachometer on the test wheel indicates test wheel speed. A pump and nozzle place a film of water beneath the test wheel. A number of the vehicles also include facility for programmable control of test distances.
Kummer et al, "Measurement of Skid Resistance," Symposium on Skid Resistance, Special Technical Pub. No. 326, ASTM (1962) and "The Penn State Road Friction Tester as Adapted to Routine Measurement of Pavement Skid Resistance," Hwg. Res. Record No. 28, NAS Hwy. Res. Bd. (1963) contain a general discussion of coefficient of friction and measurement thereof, as well as a specific device for performing such measurement. A test wheel is mounted behind a vehicle as a fifth wheel by a parallel strut arrangement. A drum brake provides selective braking of the test wheel, with the brake backing plate being restrained by a control link of the parallelogram suspension. A force transducer in the control link measures traction force. An air cylinder selectively raises and lowers the test wheel assembly, and a pump places a water film beneath the test wheel.
FAA Report No. ADS-55, "Design and Development of an Airport Runway Surface Traction Measuring Device," (1966) describes a test vehicle which includes, among other features, a positive displacement pump coupled to the vehicle drive system for maintaining uniform water film thickness beneath the test wheel independently of vehicle speed. An hydraulic transmission is employed to obtain controlled slip at a trailer-mounted test wheel. Domandl, "Measuring Tire Friction Under Slip with the Penn State Road Friction Tester," NAS Hwy. Res. Bd., Hwy. Res. Rec. No. 214 (1968), pages 34-41 also discloses test apparatus which embodies hydraulic control to obtain predetermined slip at the test wheel. Strain gages are employed for measuring forces on the parallelogram-suspended test wheel, and coefficient of friction and friction number are both indicated. Devices heretofore marketed in the U.S. under the trade designations "Skidometer BV6" and "Skidometer BV11:2" (1971) embody many principles attributed to Kullberg, including controlled slip at the test wheel and strain gages for measuring forces acting thereon. In addition, the BV11:2 apparatus featured operator switch selection of runways by number and preprogrammed runway test lengths.
Rizenbergs et al, "Skid-Test Trailer: Description, Evaluation and Adaptation," Kentucky Dept. of Hwys., Report No. 338 (1972) describes construction and evaluation of a two-wheeled test trailer heretofore marketed by applicant's assignee under the designation "Model 965A Pavement Friction Tester." Each wheel of the test trailer is coupled to a disc brake selectively powered by an hydraulic pump carried by the tow vehicle for braking the test wheels with respect to vehicle velocity. Pumps are coupled through electric clutches and gear belts to the vehicle drive shaft for applying uniform water films beneath each trailer wheel independently of vehicle speed. A four-beam two-axis strain gage force transducer is coupled to each test wheel for indicating instantaneous wheel load and traction force. The strain gages are connected in a bridge arrangement such that the horizontal and vertical forces are additive. Boyd U.S. Pat. No. 3,948,080 discloses a tire traction tester which employs a three-axis "Model 2500" version of the same transducer.
The foregoing prior art is epitomized in the disclosures of U.S. Pat. Nos. 4,098,111 and 4,212,063.